1. FIELD OF THE INVENTION
This invention relates to the field of polymer layers, particularly thermotropic liquid crystalline polymer layers.
2. DESCRIPTION OF THE PRIOR ART
The diversity of articles packaged and shipped around the globe has stimulated demand for packaging materials which provide specially-tailored performance characteristics. Some of the characteristics include strength, elasticity, resilience, adhesiveness, transparency, electro-conductivity, light-shielding, gas-barrier, heat resistance and chemical resistance. In order to achieve any one or combination of these characteristics, typically, a variety of constituent polymers thereof must be carefully balanced and tuned.
The present invention employs a layer of liquid crystal polymer (LCP) which provides a significant number of the above-desired characteristics. Specifically, LCP is well known for its dimensional stability, heat resistance, chemical stability and electrical conductive properties. LCP also has been discovered to perform as an excellent barrier to various vapors and liquids, often essential for many packaging applications. LCP offers many significant properties over other classes of polymers. First, LCP has oxygen barrier properties equal to or better than ethylene vinyl alcohol copolymer (EVOH). For example, LCP oxygen permeability at 100% RH and 23.degree. C. is 0.045 cc-mil/100 in.sup.2 -day-atm, 24 times better than that for an EVOH barrier made from EVALCA.RTM.. Typically, LCP has a moisture permeability at 90-100% RH of 0.07 g.mil/100 in.sup.2 -day-atm, versus 1 g.mil/100 in.sup.2 -day-atm for low density polyethylene (LDPE). Second, LCP has a temperature and humidity insensitive water vapor transmission rate (WVTR) several times better than EVOH and comparable to that of polyethylene at high temperatures. Third, LCP can sustain continuous service at temperatures above 200.degree. C. Fourth, LCP has a low coefficient of thermal expansion, thus minimizing potential for board warp. Fifth, LCP has excellent chemical resistance to solvents, acids, bases and oils. LCP is resistant to most solvents even at moderate to elevated temperatures ranging from 150-200.degree. C. LCP is stable when exposed to steam and hot water for extended periods of time. LCP is resistant to most to strong acids and mild bases at temperatures ranging from 50-100.degree. C. LCP also exhibits excellent resistance when exposed to gamma radiation. Sixth, LCP delivers excellent retention of aromas and flavors. Seventh, LCP has excellent resistance to surface staining. Eighth, LCP provides excellent mechanical strength, much higher than EVOH or nylon. Ninth, LCP exhibits excellent heat degradation resistance, both in solid and melt state.
LCPs fall into a class of high-performance thermotropic plastic materials which have very rigid, rod-like molecules and are highly ordered in both the melt and solid state. In the melt state, LCP flows easily and may be processed with almost all techniques commonly associated with thermoplastics. LCP offers the advantage of low processing temperatures ranging from 550-650.degree. F. In some cases, however, special processing equipment may be necessary. The rod-like nature of LCP molecules results in a profile of molecular orientation that resembles the physical orientation of the fibers in a reinforced thermoplastic. This flow behavior causes a self reinforcing effect that results in exceptional flexural strength and modulus, as well as good tensile performance, that improves bulge resistance.
LCP is a well-known material that has been used in many diverse applications, but not as a stand-alone barrier layer or in conjunction with a simple tie layer on a substrate. For example, in U.S. Pat. No. 5,248,530, issued Sep. 28, 1993, to Jester et al., a co-extruded first LCP layer is laminated to a second LCP layer having a lower melting temperature. The second LCP layer serves as a tie layer so that the composite layer may be bonded to another material.
U.S. Pat. No. 5,364,669, issued Nov. 15, 1994, to Sumida et al., describes a composite film comprising an LCP layer and a thermoplastic polymer layer laminated thereto. Similar to the Jester et al. invention, the thermoplastic layer of Sumida et al. functions as a complex tie layer.
U.S. Pat. No. 5,405,565, issued Apr. 11, 1995, to Sumida et al. describes a laminated film comprising an LCP layer through which is laminated a thermoplastic polymer layer which is adhesive to the LCP layer. This composite layer then is laminated onto a thermoplastic polymer layer which is not adhesive to the LCP layer.
The above demonstrates the need for employing LCP, either alone or with a simple tie layer, as a barrier layer on substrate materials for packaging cartons and trays.
None of the above references, take alone or in combination, are seen as teaching or suggesting the presently claimed invention.